[DocumentIdDB] Silence non-error debug message, warn on errors

Summary:
MDB_NOTFOUND is not an error, remove the message. In case any other
error occurs, actually print a warning.

While at it, reserve enough space in the returned vector.

Reviewers: Baloo, Frameworks, ngraham, astippich, poboiko

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D20329